Melatonin (N-acetyl-5-methoxytryptamine) is a hormone primarily synthesized and secreted by the pineal gland and has been shown to regulate mammalian circadian rhythms and reproductive functions (Kennaway and Wright, 2002; Guerrero and Reiter, 2002). Melatonin is a lipophilic hormone widely distributed throughout the nervous system, blood and peripheral tissues of mammals (Vaughan et al., 1978; Rogawski et al., 1979). It exerts its biological effects through interaction with specific melatonin receptors. Two human melatonin receptors, MT1 and MT2, have been identified and cloned, both possessing a similar binding affinity for melatonin (Reppert et al., 1994, 1995). These receptors are G protein-coupled receptors, indicating a triggering mechanism via G protein mediated signaling pathways. The G proteins, in turn, directly or indirectly (via a second messenger) regulate various effector systems. Due to different classes of G proteins that are able to mediate the downstream signaling pathways of melatonin receptors (New et al, 2003), MT1 and MT2 can trigger many distinct signal transduction cascades, leading to the activation of various unique cellular responses (Witt-Enderby et al., 2003).
The melatonin receptors MT1 and MT2 are expressed in a wide variety of tissues within the body. The MT1 receptor is expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus within the brain, the cardiac vessels and various regions of the brain and peripheral tissues. MT1 receptors are also found in normal and malignant breast tissue (Dillon et al, 2002). MT2 receptors are more localized and found in the cerebellum and SCN within the brain, retina, ovary, kidneys and cardiac vessels. Both receptors are thought to play a role in mediating the sleep/wake cycle by modulating the body's circadian rhythms (Dubocovich et al., 1998) but while the MT1 receptor is thought to regulate sleepiness, the MT2 receptor is thought to help regulate sleep-wake cycles, and while MT1 receptors constrict cardiac vessels, MT2 receptors dilate them (Doolen et al, 1998). Furthermore, MT2 receptors are involved in inflammatory responses. Therefore, with the wide distribution of both receptors in many different tissues within the body, it is not surprising that melatonin is involved in numerous physiological processes of the body.
Melatonin has been implicated in the regulation of a number of physiological processes (Pandi-Perumal et al, 2006) and thus, has been used in the treatment of many biological disorders. It is widely used in the treatment of chronobiological disorders such as seasonal affective disorders (SAD) (Rosenthal et al., 1986), primary and secondary insomnia, and sleep disorders caused by blindness (Nakagawa et al., 1992), shift work (Sack et al., 1992) and jet lag (Arendt et al., 1991). Melatonin has also been linked to retinal physiology (Dubocovich et al., 1997), blood pressure regulation (Doolen et al., 1998), and in the regulation of the immune system (Guerrero and Reiter, 2002), and inflammation (Cuzzocrea and Reiter, 2002). In addition, melatonin has been shown to have a strong effect against cancer and tumor growth (Reiter, 2003; Blask et al, 2002). Furthermore, in numerous recent investigations, melatonin has been reported to inhibit the growth and progression of a variety of tumor cells including breast cancer, ovarian carcinoma, endometrial carcinoma, melanoma, prostate tumor and intestinal tumor cells (Pandi-Perumal et al. 2006).
Recent evidence also indicate that melatonin may be beneficial in the treatment of psychiatric disorders (bipolar, depression and anxiety disorders, schizophrenia, epilepsy and epileptic seizures), neurodegenerative diseases (Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease, amyotrophic lateral sclerosis, muscular sclerosis), stroke and neuroendocrine disorders (peptic ulceration, psoriasis). A recent study offers experimental evidence supporting melatonin as a promising therapeutic agent in the treatment of PD (Khaldy et al., 2003). Melatonin has also been shown to confer a protective effect against epilepsy in humans (Molina-Carballo et al., 1997) and rats (Bikjdaouene et al., 2003) and this effect is thought to occur by increasing GABAergic neurotransmission (Acuna-Castroviejo et al., 1995). Melatonin also has shown to confer beneficial effects in Alzheimer's patients. A recent report has indicated that patients with AD exhibit reduced expression of the melatonin receptor subtype MT2 (Savaskan et al., 2005). Administration of melatonin to AD patients have garnered promising results, including improved cognitive function, improved sleep and a significant reduction in the rate of disease progression (Maurizi, 2001; Cardinali, 2003).
Melatonin also plays a role in neuroprotection. Recent reports demonstrate the involvement of melatonin in neuroprotection after an acute cerebral ischemic stroke. Studies on animal models of focal cerebral ischemia indicate that melatonin confers a marked neuroprotective effect (Kilic et al., 2005; Lee et al., 2005; Macleod et al., 2005) and exerts anti-inflammatory effects (Pei and Cheung, 2004), and results have suggested that melatonin may be a good candidate as a neuroprotective drug for stroke in humans (Macleod et al., 2005). Melatonin's neuroprotective action is thought to occur via its antioxidant and free radical scavenging activity (Lee et al., 2005).
Despite the modulatory effects of melatonin on various effector systems, its use in clinical applications has been limited by its short biological half-life, poor oral bioavailability and ubiquitous action (Uchikawa et al., 2002). Therefore, in recent years, much work has been undertaken to identify or develop melatonergic agonists that are more metabolically stable than melatonin, exhibit higher affinity to the MT1 and MT2 receptors, and most importantly, show selectivity towards one receptor sub-type over the other. Melatonergic agonists of synthetic origin mimic the signal cascade generated by melatonin by binding to the melatonin receptors, MT1 and MT2. They are of immense value for the physiological study of the different melatonin receptor subtypes and help delineate the manner by which melatonin modulates and triggers other effector systems.
As melatonin has been shown to mediate sleep-wake cycles by broadly targeting receptors, melatonergic agonists that demonstrate melatonergic properties and high affinity and selectivity for one or both of the melatonergic receptor sub-types are valuable both as therapeutic agents and research tools. They are promising therapeutic agents for the treatment of insomnia and circadian-related disorders, as well as in the treatment of other disorders related to melatonin as mentioned above.
In addition to the hormone melatonin, there are several other known melatonergic ligands that interact with the melatonin receptors. These include 2-iodomelatonin and N-acetylserotonin, which, like melatonin, bind to both MT1 and MT2 receptors albeit with different affinities. These agonists have been used for pharmacological evaluation of the receptor subtypes but their use has been limited by their lack of selectivity between the two receptor subtypes. Further, they are not promising drug candidates. Ligands to the melatonin receptors which exhibit selectivity towards one of the receptors are of greater pharmacological and therapeutic value. Therefore, in recent years, many different research groups have used medicinal chemistry to synthesize agonists that exhibit greater selectivity and higher binding affinities.
Most of the known melatonin receptor ligands are developed based on the indole structure of melatonin. Critical features include the 5-methoxyl group (Chong et al., 1993; Mor et al., 1998; Sicsic et al., 1997; Sugden et al., 1995) and the 3-ethyl amide chain (Grol and Jansen, 1996). The relative distance and conformations of the methoxyl and the amide group on melatonin are some of the critical factors determining the potency and efficacy of the ligands. The alkyl chain attached to the amide carbonyl group has a limitation of <3 carbons in length in order to retain a high affinity (Sugden et al., 1995). Tolerance and specificity of C2 and C6 substitutions have been studied intensively (Spadoni et al., 1993; Sugden et al., 1995). Other melatonin ligand structures adopt various heterocyclic scaffolds such as tricyclic and tetracyclic indole-based rings, indane, naphthalene, tetraline, quinoline, benzoxazole, benzofuran, etc. (Zlotos, 2005).
In plants, alkaloids are involved in defense mechanisms against herbivores and pathogens. Published reports have indicated that alkaloids in plants confer neuroprotective activities. They have also been reported to possess antiviral, antimicrobial and immunomodulating activities (Hudson, 1990; Huang, 1999), as well as potent anti-tumor agents (Cragg et al., 1997; Cragg and Phil, 1999).
Although U.S. application Ser. Nos. 10/135,247 and 10/738,964 as well as PCT Application Publications WO/2005/075431 A1 and WO/2005/075432 A1 have disclosed some isoquinolone derivatives, prior to the present invention, the applicants are not aware of any isoquinolone derivatives that possess melatonin receptor agonist activity.
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